1. Field of the Invention
The present invention relates to a method for fabricating a three-dimensional photonic crystal.
2. Description of the Related Art
A photonic crystal is a structural body in which a refraction index of the structure material is periodically distributed, and is an artificial material which can realize a novel function by only devising a structural design.
The greatest feature of the photonic crystal includes that the photonic crystal has a so-called photonic band gap that is a region through which a particular electromagnetic wave cannot pass, and is formed by a difference of refractive indices between components and the periodicity of its structure. An energy level (defect level) caused by a defect is formed in the photonic band gap, by introducing an appropriate defect into the distribution of the refractive index in the photonic crystal. Thereby, the photonic crystal can freely control an electromagnetic wave. Besides, the photonic crystal can further miniaturize the size of a device using the photonic crystal than that of a conventional device.
Furthermore, a three-dimensional photonic crystal has refractive indices of structure materials distributed with three-dimensional periodicity and hardly leaks an electromagnetic wave that exists in a position of the defect outside.
In other words, the three-dimensional photonic crystal is the most suitable material for controlling the transmission of the electromagnetic wave.
Among representative structures of such a three-dimensional photonic crystal, one known structure is a wood pile structure (or rod pile structure) which is disclosed in U.S. Pat. No. 5,335,240. The wood pile structure in the three-dimensional photonic crystal has a structure as is illustrated in FIG. 2.
In FIG. 2, reference numeral 70 denotes a three-dimensional periodic structure in which a plurality of stripe layers are sequentially stacked, in which a plurality of rods 40 are arranged in parallel to each other periodically with a predetermined period in the plane.
Specifically, the three-dimensional periodic structure 70 has four stripe layers including: the first stripe layer in which a plurality of rods are arranged in parallel to each other periodically with a predetermined period in the plane; the second stripe layer in which rods are stacked on the first stripe layer so that the direction can be orthogonal to each rod belonging to the first stripe layer; the third stripe layer in which rods are stacked on the second stripe layer so that the direction can be parallel to each rod belonging to the first stripe layer and the period can be deviated from that in the first stripe layer by only a half of the period in the plane; and the fourth stripe layer in which rods are stacked on the third stripe layer so that the direction can be parallel to each rod belonging to the second stripe layer and the period can be deviated from that in the second stripe layer by only a half of the period in the plane. Then, the three-dimensional periodic structure 70 is structured by sequentially stacking a plurality of sets each of which is formed of the four stripe layers.
The period of the structure of the photonic crystal is approximately a half of a wavelength of an electromagnetic wave to be controlled.
However, the three-dimensional photonic crystal generally has a complicated structure, and needs a complicated process and many steps, though being expected to have ideal device properties.
Conventional methods for fabricating a three-dimensional photonic crystal having a wood pile structure include a method of heat-bonding different members to each other with a laminating technology described below, which is proposed in Japanese Patent Laid-Open No. 2004-219688.
The heat bonding method includes: firstly forming an array of rods which are arranged in parallel to a stripe layer provided on a substrate with a predetermined period in the plane; joining the above described stripe layers with the heat bonding method while aligning the interlayer; and removing the substrate of one stripe layer.
A wood pile structure having layers corresponding to the number of the joining steps can be obtained by repeating the above steps.
The laminating technology described above shall enable the production of the three-dimensional photonic crystal having a relatively complicated structure.
The methods for fabricating the three-dimensional photonic crystal also include the following stacking technology as is disclosed in U.S. Pat. No. 5,998,298.
The above stacking technology includes: forming a thin film; processing the thin film; then, forming a sacrificial layer; and flattening the sacrificial layer until the processed thin film comes to the surface, by polishing the sacrificial layer with a CMP (Chemical Mechanical Polishing) technology.
By repeating the above described process, the stacking technology enables the production of the three-dimensional photonic crystal with higher accuracy than a stacking method which does not employ the sacrificial layer.
On the other hand, as for a conventional method of processing a thin film, U.S. Pat. No. 5,236,457 discloses a method of forming a pattern and a method of fabricating a semiconductor element, as will be described below.
The above method enables the thin film to be processed by a step of injecting an ion beam described below and a step of dry-etching a material to be etched.
Specifically, the step of injecting the ion beam includes: injecting ions into the thin film while changing an injection position on the material to be etched, to which the ion beam is focused, and changing at least one of accelerating voltage, a type of an atom of an ion, and a valence of an ion; and forming a peak region of ion concentration in a depth direction of the material to be etched.
The dry etching step includes dry-etching the material to be etched with such an etching gas as to hardly etch the peak region of the ion concentration because of being inhibited by the ion. The thin film is processed by the above steps.